New patch sets on integration failure causes Coin to re-build artifacts

Coin stores all artifacts it produces. So if a qtbase build fails on autotests but the compilation passes, the next time you restage the commit, it will download the built artifacts and proceed right to testing.

Now as Gerrit produces new patch sets each time a commit is staged, the next time Coin can't know if the sources are different or not, as it only sees a change, it will rebuild qtbase every time.